1. an extreme state of adversity; the lowest point of anything; [syn: nadir, low-water mark]

2. the point below the observer that is directly opposite the zenith on the imaginary sphere against which celestial bodies appear to be projected;

The Collaborative International Dictionary of English v.0.48:

Nadir \Na"dir\ (n[=a]"d[~e]r), n. [F., Sp., & It. nadir; all fr.
Ar. naz[imac]ru's samt nadir, prop., the point opposite the
zenith (as samt), in which naz[imac]r means alike,
corresponding to. Cf. Azimuth, Zenith.]
1. That point of the heavens, or lower hemisphere, directly
opposite the zenith; the inferior pole of the horizon; the
point of the celestial sphere directly under the place
where we stand.
[1913 Webster]
2. The lowest point; the time of greatest depression.
[1913 Webster]
The seventh century is the nadir of the human mind
in Europe. --Hallam.
[1913 Webster]
Nadir of the sun (Astron.), the axis of the conical shadow
projected by the earth. --Crabb.
[1913 Webster]